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spi-bi-wire problem

Dan-o
Intellectual 895 points

Other Parts Discussed in Thread: TPS3801

I have a programming board which takes the 14 pin JTAG from a MSP-FET430UIF and puts the 4 wire JTAG signals on a different connector along with some serial data lines and 3.3V power for the target board. I have modified this board to use spi-bi-wire.

Programming board setup -

The target board connector which has the 4 wire JTAG signals / serial lines / 3.3V from programming board is floating.

3.3V on the programming board powers the target, and is also jumpered to pin 4 (VCC from target) on the 14pin JTAG conn.  The target has a 3.3V output DC/DC converter which powers the MSP430 so I think this should be ok.

Pin 1 (TDO/TDI) of 14pin JTAG goes to the \RST pin on target board MSP430

Pin 7 (TCK) of 14pin JTAG goes to TEST/SBWTCK pin on target board MSP430

My target board in question has a voltage supervisor (TPS3801) on the MSP430 /RST pin which will hold the /RST pin low for approx 200ms after VCC has risen.

When I try to download and debug in IAR, the board does a reset but IAR fails to connect. I have messed with Target VCC settling time in the project options to no avail. Any ideas???

  • 0
    Guru 227245 points

    SBW uses higher frequencies on teh JTAG lines and also requires short signal propagation times, as the data signal is bi-directional.
    So long cable or a larger capacitance (including the pulldown capacitor on RST pin) will disturb the communication.

  • 0
    Guru 12160 points

    Dan-o,

    I looked at the data sheet for the TPS3801 and it appears the device uses a push-pull driver to control /RST.  Is that right?

    If so, then it's not compatible with SBW.  You will need to find a supervisor with an open-drain or open-collector output.  This will allow the JTAG tool to control the pin as SBWTDIO.

    Jeff

  • 0 in reply to Jens-Michael Gross
    Intellectual 895 points

    thanks for the help. hadn't got a chance to update my post yet but I figured it out. Turns out it was the TPS3801 voltage supervisor. its a push pull and the spi-bi-wire reset couldn't overcome the output. The pin 9 RST for 4 wire JTAG had no problem pulling it down.

  • 0 in reply to Jeff Tenney
    Intellectual 895 points

    you beat me to the post :) thanks for the help

  • 0 in reply to Jeff Tenney
    Guru 227245 points

    Jeff Tenney said:
    You will need to find a supervisor with an open-drain or open-collector output. 

    Not necessarily. There are two workarounds:

    First, a diode (and a pullup, but it's already there usually). This will turn the supervisor in an 'open-collector' type.

    Second, a series resistor that is larger than the series resistor that connects the FET. This way, the FET can override the supervisor signal  The supervisor is only active as long as the FET is not connected and pullign the pin high or low actively.

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